Why are there earthworms in the swimming pool?

Answer 1

You find earthworms in your pool in the mornings likely because earthworms are primarily active at night and end up wandering into the pool and become unable to get out, so you don't notice them until morning.

Answer 2

They crawl out of the ground and accidentally end up in cement. They continue to squirm until they fall into the pool... and drown :-(